Head positioning systems for controlling the position of data heads in disc drives are generally known in the prior art and are described, for example, in the Hewlett Packard Journal, Vol. 35, No. 1, January 1984, incorporated herein by reference. These systems operate on servo code encoded on concentric tracks of magnetic discs. This code is used in both track centering of the heads and head movement across the tracks to a selected track to be read on a selected disc (seek) during disc drive operation. Such a servo system is also disclosed, for example, in U.S. Pat. No. 4,115,823.
Information which is encoded on tracks of a storage medium, such as a magnetic disc, is often shifted radially thereon as a result of exposure to environmental factors such as aging, temperature cycling, shock, and the like. This problem of information shift (relative shift) can also be caused by tilt of the disc spin axis or by tilt of the rotary axis of an arm stack relative to the planes of the disc surfaces of a stack of discs. In both cases there is relative movement between the transducer heads and the recording tracks on the separate discs.
As a result of this shift in encoded information with respect to a head position and the fact that the corresponding servo code on a dedicated track of the disc is by definition the reference code and thus is not seen as being shifted, an initial dedicated head, track center offset error, if uncompensated, may result in a track count error at the end of a track seeking movement of the dedicated transducer head. More specifically, since the seek servo processor uses dedicated head position offset information from the dedicated track surface, precise information as to the location of the dedicated head from a given track center in magnitude and direction is needed to develop an unambiguous, servo driving voltage and to unambiguously indicate whether or not the first track crossing is a valid count in seeking a designated target track.
Various approaches have been taken previously in dealing with such problems. Some of these, as indicated by U.S. Pat. No. 4,115,823 have addressed this problem. U.S. Pat. No. 4,115,823, using normal and quadrature phase voltages derived from voltages from a servo head, uses the quadrature position error signal to extend the linear region of the normal error signal beyond the .+-.1/4 track range. The present invention provides further improvement extending track offset resolution into the fourth quarter, or to nearly a full track.